documentation/Makefile: support to generate localized docs.

New variable "LN" indicates language code for a given manual.
If you use "LN", before to xml pdf and/or html or to verify xml,
l10n target will generate localized xml file with po/$(LN).po,
and result pdf/html are localized.

Example to generate Japanese Quick Start Guide:
  make LN=ja DOC=yocto-project-qs
documentation/Makefile: add target of updatepo

updatepo target will update po files for all languages.
documentation/Makefile: update comments.
documentation/Makefile: updatepo update pot file too.
documentation/yocto-project-qs: add pot/po files for QS.
yocto-project-qs/po/ja.po: update japanese translation.